Anthrax

ANTHRAX

Synonyms

• Splenic fever, Milzbrand, Charbon, Wool sorters disease, and Malignant carbuncle.

Introduction

• It is a peracute, acute and septicaemic disease of cattle and sheep leading to sudden death.
• The extensive enlargement of the spleen given this disease the name splenic fever.
• It is a zoonotic disease spread from infected animals to human during processing of wool and hides.

Etiology  

• Bacillus anthracis is a pathogenic strain causing anthrax in both human and animals.
• It is a non-motile, capsulated, aerobic, spore-forming, gram-positive, organism.
• Appears as straight, very large rod-shaped, arranged in a small short chain.
• The organism has two virulent factors such as a capsule containing poly D-glutamic acid, which resists the phagocytosis induced by the host immune cells and is coded by the virulent plasmid pxo2, and the triple toxins such as edema factor-1, lethal factor-11, and the protective antigen factor- 111 and these are coded by the plasmid pXo1.

The organism produces spores when exposed to the external environment  and can live for a prolonged period outside the host in the environment.

EPIDEMIOLOGY

Prevalence of infection

• The disease is reported to be recognized from sub-Saharan Africa and has worldwide distribution.
• The prevalence of the disease depends upon the nature of the soil and climatic condition, because, the climatic parameter helps to forecast the “anthrax year”.
• If the infection is restricted to a particular area is called as “anthrax belts” or enzootic region. During heavy and prolonged rainfall, following a draught, or dry summer, in warm weather and in the temperature at which (>60F /15 ° C) formation of spores, vegetative proliferation with the production of disease or the outbreak occurs.
• In African countries, every summer with heavy rainfall is followed by a devastating occurrence of anthrax.
• In India this is enzootic.

Economic impact

• Mortality, reduced milk yield  and the expenses incurred for vaccination leads to severe economic loss.

Predisposing factors

• pH of the infected soil, infected wool, materials, hides and other materials.
• In wet soil, and in stagnant water spores are concentrated and suspended.
• Alkaline soil (pH 9.0) under warm climate preserves the organism to develop anthrax but acid soil destroys the organism.
• Blood transfusion may also spread the disease.
• Contaminated bone meal, pasture contaminated with tannery effluents, fodders grew on infected soil, turning up of soil, blood and other discharges from infected animals are a major source of infection.
• Calcium rich alluvial soil, organic and poorly drained soils have the risk of causing endemic infection.

·         Source of infection

·         Contaminated soil, environment,  infected excretions  and secretions from animals.

TRANSMISSION

Transmission

• Ingestion, inhalation and contamination of the wound by spores are the prime modes of spread.
• Injury to oral mucosa permits the invasion of the organism.
• Inhalation plays a less important role in animals contrast to more spread in human being and is responsible for causing wool sorters disease.
• Mechanical transmission through Tabanid fly bite occurs.

Host affected

• Cattle and sheep are highly susceptible as compared to goats and horses.
• Human is less susceptible than cattle.
• Pigs, dogs and cats are relatively resistant.
• But once affected the case fatality rate is high in farm animals and pigs.
• Hippopotamus, cape buffalo and elephants die in large numbers due to anthrax.
• Persons working in wool and hide industries are mostly affected.
• Predators acts as a carrier for this infection.

PATHOGENESIS

• Discharges and materials containing anthrax bacilli  when  exposed to air, spores are produced.
• The spores are very resistant in the external environment but the un-germinated spores are considered not harmful it becomes a vegetative form inside the host.
• The organism is resistant to phagocytosis due to the presence of poly D glutamic acid-containing capsule, and triple toxin factors such as edema factor-I, lethal factor-II and protective antigen-III.
• Primarily, spores proliferate in the regional draining lymph nodes, subsequently, travels via lymphatic vessels and reach the bloodstream and cause septicemia followed by massive invasion into all body tissues.
• The lethal toxin of B.anthracis causes edema, tissue damage, and death as a result of shock, acute renal failure, and terminal anoxia. The blockage of capillaries initiating death is commonly known as the “Log Jam” theory.
• As per the evidence, at least 3x106 bacteria/ml blood is necessary for causing death.
• In pigs, localization of the organism occurs in the lymph nodes of the throat following invasion through the upper part of the digestive tract eventually lead to fatal septicemia. 
• Shock acute renal failure and terminal anoxia lead to death.

CLINICAL SIGNS AND ZOONOSIS

Cattle and sheep

Peracute form

• Sudden death without any premonitory signs. Upon close observation following signs will be seen.  
• Fever, muscle tremor, dyspnoea, congestion of mucous membranes are noticed.
• Animal collapses and die due to terminal convulsions.
• After death, oozing of blood from nostrils, mouth, anus and vulva is noticed.

Acute form

• Incubation period of the disease is 1-2 weeks.
• The course of this form is about 48 hours.
• Fever (42°C), severe depression, listlessness, congestion, and hemorrhage of the deep mucous membrane, increased heart rate, and rapid respiration observed.
• Affected cattle with mouth breathing due to oxygen hunger.
• Abortion, the decrease in milk yield, blood-stained or deep yellow color milk, diarrhea, local edema of the tongue, throat, sternum, perineum and flank occur.

Pigs

• Acutely and sub-acutely infected animals show  high temperature, appears dull, anorectic with distinct inflammatory edema in throat and face.
• Painless swelling causes obstruction which leads to  swallowing and respiration difficulty.
• Blood stained froth present at the mouth.
• Petechial hemorrhages of skin, dysentery without any edema of throat occurs.

Pulmonary form

• In baby pigs, it develops lobar pneumonia and exudative pleurisy through inhalation of dust which leads to death in a day or two.
• Occasionally, affected individuals may be alive for several days after infection.

Horse

• Always horses are affected with acute  infection by ingestion and lead to septicemia, enteritis, and colic.
• Painful, edematous and subcutaneous swellings appear in the throat over the lower neck, the floor of the thorax, abdomen, prepuce and mammary gland.
• There is a high fever, severe depression, dyspnoea due to swelling of the throat or colic due to intestinal irritation.
• The course of the disease is about 48-96hours.

Zoonosis

• It is a fatal disease of human beings.
• B.anthracis is an agent of bioterrorism and listed as category A disease by OIE.

Occupational hazard by inhalation of spores occurs mostly while sorting out wool and processing of hides.

NECROPSY FINDINGS

• Absence of rigor mortis with gaseous decomposition, quick assumption of the characteristic sawhorse posture.
• Un-clotted tarry colored blood oozes out through natural orifices.
• There is a soft and enlarged spleen with a blackberry jam consistency and is an important feature of anthrax.
• The carcass suspicious of anthrax should not be opened before confirmation made by peripheral blood smear examination.

In horses and pigs enlargement of local lymph nodes, lesions in the soft tissues of the neck and pharynx are important and it is called a pharyngeal form of anthrax.

DIAGNOSIS

Diagnosis

• Based on clinical signs and necropsy findings.
• Isolation and identification of the organism.
• Haematological and blood chemistry examinations should not be conducted because of the risk for human exposure.
• The square-ended blue bacilli (chain) with metachromatic pink capsule detected by microscopic examination after staining with ripened polychrome methylene blue stain is called as 'McFadyean's reaction'.
• In early stages bacilli may not be present in the blood, so, culture can be done or the materials like infected fluid can be injected into guinea pigs would give proper diagnostic results.
• Fluorescent antibody technique using monoclonal antibody can be done to detect the disease.
• In decomposed carcass vegetative bacilli may not be present, in such a condition diagnosis is difficult so that the protective antigen is detected by the immuno-chromatography test. It has high specificity but does not give positive results in recently vaccinated cattle.
• Isolation of bacilli from infected soil is difficult but real-time quantitative PCR and Nested PCR have been found highly sensitive.
• In human cutaneous anthrax is diagnosed by Immunohistochemical detection of the bacilli in the skin biopsies.
• Gelatin stab culture: The growth resembles an 'inverted fir tree appearance' with slow liquefaction commencing from the top.
• Nutrient agar media: The 5% blood agar show 'medusa head colonies'.

	B.anthracis Gram stain. Cells : Squared end Endospores: Ellipsoidal shape, Central in the sporangium. Spores are highly refractile to light and resistant to staining.
	B. anthracis Incubation at 5% CO2 tension increase the production of poly-D-glutamyl acid production by capsule thereby mucoid colonies.

	B. anthracis McFadyean's reaction: Cells in short chains with plenty of disintegrated capsular material. White blood cells stained blue.
	B.anthracis: Medusa Head Colonies.
	Anthrax Spore Vaccine It is a nonpathogenic, nonencapsulated variant vaccine strain Sterne 34F2 of B. anthracis spore vaccine in saponin, in India.

• Ascoli's test: To demonstrate the antigen in the severely decayed carcass and the skin  in which tube containing positive serum is overlayed with suspected sample develops a white precipitation ring at the junction of the two liquids.

Sample collection

• Unopened carcass, blood, edema fluid in sealed leak-proof containers and direct smear from the tip of the ears.
• In live animals, the organism may be detected in a stained peripheral blood smear, edema fluid, lymph node needle puncture smear, amputated tail or ears and jugular vein aspiration etc.
• If anthrax is suspected then shipping of diagnostic samples via air or courier is not permitted.

Differential diagnosis

• Lightening strike.
• Peracute blackleg.
• Malignant edema.
• Bacillary hemoglobinuria.
• Hypomagnesemia.

TREATMENT

Treatment

• Recovery occurs when the infection is identified in the early stages and treated with penicillin 20,000 IU /kg BW twice daily for 3 days.
• Streptomycin 8-10g/day in two divided doses IM for cattle is effective.
• Oxytetracycline 5-10mg /kg/day IV or IM can be given.

Antiserum is given daily for 5 days, however, it is expensive.